1. Field of the Invention
The present invention relates generally to measurement of position or location by means of electromagnetic signaling and especially to a system and method for evaluating the position and associated data of a transmitter using a plurality of low frequency receivers.
2. Background of the Invention
A variety of prior art solutions seek to track the positions of shipping containers. Especially challenging are shipping containers concealed in or obscured by a stack of containers as may be encountered in a shipping yard or aboard ship. These prior art solutions include global positioning systems (GPS) and transponder ranging type systems. GPS uses weak signals that do not propagate well in non-line-of-sight environments.
Similarly, time-of-flight or transponder ranging works poorly in the non-line-of-sight environment between and around shipping containers. Delay spreads in shipping container environments often exceed 1 μs, making it extremely difficult to time signals to an accuracy of a few nanoseconds and thus range to an accuracy on the order of a meter, as described in: “Technical Report: Naval Total Asset Visibility (NTAV) Tests on the SS Curtiss, Port Hueneme, Calif., 25 Sep.-6 Oct. 2000,” by Gunderson et al.
Practitioners of the RF arts generally recognize that electromagnetic waves propagate best when a waveguide, or propagation environment has gaps at least equal to roughly a half wavelength in dimension at the frequency of propagation [J. C. Slater, Microwave Transmission, (New York: McGraw-Hill Book Company, Inc.), 1942, pp. 124-129]. Typical gap sizes between stacked containers are on the order of about 10 cm. Thus, it is generally believed that significant electromagnetic wave propagation cannot occur between stacked containers when the frequency is significantly less than 1.5 GHz.
Traditional RF practice is thus caught between the Scylla of the requirement for high frequencies to support electromagnetic wave propagation in narrow gaps and the Charybdis of high multipath and delay spreads that render traditional time-of-flight positioning technology ineffective.
Modern supply chain management practices demand precise and timely information on the status of goods in commerce. Additionally, keeping containers secure and safe from tampering is a priority throughout the supply chain. Furthermore, there are some 15 million shipping containers at present in the world. Any one of these containers could arrive in a friendly port carrying a deadly cargo. There is an immediate and significant need for a system to secure and track shipping containers. Thus there is a corresponding need for an electromagnetic communication and tracking technology that can effectively track and monitor shipping containers alone or in stacks; in yards or aboard ship and can overcome the problems associated with traditional RF technologies.